Expiration dating for Standard and Reference preparations has often been assigned arbitrarily; since references and standards are a critical part of release and research activities of the Center, a program to characterize the chemical and immunological stability of these preparations is continuing. Using various immunological methods, including electrophoresis, immunoblotting, ELISA, cell assays and animal tests, toxins and antitoxins are periodically examined qualitatively and quantitatively. Information related to the stability of Diphtheria toxin has been incorporated into the new assay method for Diphtheria Antitoxin. This new assay method, consisting of a cell survival assay, has been employed to examine the retention of potency of both toxin and antitoxin. Expiration dating for current glycerinated Diphtheria and Tetanus antitoxin standards has been extended; a freeze-dried Diphtheria antitoxin is being evaluated. Based on collected information, a new preparation method was implemented for the preparation of Control Tetanus Toxin. This preparation, T-3, replaced the old T-1 control after manufacturers and CBER testing found the new toxin preparation to respond similarly to T-1 in product release testing.